The invention herein relates to rotary pressure precoat filters.
Rotary pressure precoat filters work on the same principle as the more common rotary vacuum precoat filters, i.e., the use of a pressure differential across the precoat layer to force the filtrate through the precoat and filter septum leaving the solid material collected in a layer on the top of the precoat layer. The common rotary vacuum precoat filter is limited to the pressure differential between ambient air pressure on the outside of the filter and the reduced pressure on the inside, a differential theoretically as high as 14.7 psi (1 atm) but which in practice is somewhat less. The rotary pressure precoat filter, on the other hand, operates inside a pressure vessel so that the external pressure on the filter can be substantially higher than ambient pressure. Thus the pressure differential across the filter can be many times higher than that obtainable with the rotary vacuum precoat filter. Pressure differentials across a rotary pressure precoat filter can be as high as several hundred psi (several tens of atmospheres). This permits filtration at reasonable flow rates of many viscous and volatile liquids which cannot practically be filtered by rotary vacuum precoat filters.
Rotary precoat filters, both vacuum and pressure, include means for continuously removing the collected solids and a thin top layer of the precoat as the filter revolves. There is therefore always a short section of precoat layer which is not covered with the cake of filtered solids and therefore is less resistant to pressure on the surface of the filter drum between the location of the removal means and the point where the drum surface reenters the slurry vat. The reduced gas resistance through this segment of the precoat serves to reduce the efficiency of the filter for the reduced pressure differential creates a tendency for the driving gas to "bypass" other sections of the filter cake where there is greater resistance and pass through this zone of lower resistance. To compensate for this effect and maintain adequate pressure differential unduly large quantities of gas must be used.
In a rotary vacuum precoat filter this "bypassing effect" is minimal because the total pressure differential across the filter cake is small and thus a slight reduction at one point does not cause significant practical complications. With a rotary pressure precoat filter, however, where the pressure differential across the filter is large, the bypassing effect caused by the reduced pressure differential can result in a marked increase in volume of gas recirculation. Consequently it would be of definite interest to have a rotary pressure precoat filter which incorporates means to reduce the bypassing effect. Such means must not detrimentally affect the efficiency of the filter.
Conventional rotary pressure precoat filters, being sealed in pressure vessels, present significant problems of maintenance, particularly for moving parts such as bearings and seals. Unless one uses elaborate and complicated remotely controlled devices for such services as lubrication and cleaning, routine maintenance requires shutting down and depressurizing the unit so that the pressure vessel can be opened and the maintenance performed. The deficiencies of this system and the costs of both the labor involved and the lost production are evident. It would therefore be distinctly advantageous to have a rotary pressure precoat filter system wherein those components such as bearings and seals which most commonly require periodic maintenance are readily accessible for such maintenance without the necessity of depressurizing or otherwise shutting down operation of the system.